ATMOSPHERIC MASS-LOSS FROM TITAN BY SPUTTERING

Previous investigations by the authors of non-thermal escape processes such as dissociative recombination of N-2(+) ions and impact dissocia tion of N, by magnetospheric electron precipitation of solar e.u.v. ph otons have shown nitrogen escape rates much lower than originally esti mated from Voyager 1 observations, according to which 10% of Titan's a tmospheric mass could have been lost over the past 4.5 billion years. Here, the erosion of Titan's nitrogen atmosphere resulting from sputte ring, due to energetic particles occurring when Titan is either in the solar wind or in Saturn's magnetosphere, is investigated. Sputtering from Titan's nitrogen atmosphere leads to an escape rate of about 3 x 10(25) N atoms s(-1) when solar wind protons are the responsible cause , and to about 7 x 10(26) N atoms s(-1) when magnetospheric par ticles in Saturn's magnetosphere are considered. Thus, the total atmospheric mass loss over the age of the Solar System by sputtering would amount to about 20% of the present atmospheric mass of Titan. In contrast to impact ionization/dissociation, there will be no extended atomic nitr ogen corona present above Titan, since for sputtering the excess energ y leads primarily to escape and supply to the Saturn system rather tha n to ballistic orbits. Sputtering will yield, however, a small N-2 cor ona of Titan.